1. Field of the Invention
The present invention relates to a method for dewatering sewage sludge by a process wherein only organic materials are selectively purified and recovered from a suspension of hydrophilic inorganic materials and organic materials present in sewage sludge based on the hydrophobicity of carbohydrates. More particularly, the present invention relates to a method for dewatering sewage sludge by sludge-coal-oil co-agglomeration (hereinafter, abbreviated as “SOCA”) which comprises lipophilically conditioning the surface of sludge and mixing the surface-conditioned sludge with fine oil particles to form sludge-oil agglomerates in the form of small spheres. The sludge-oil agglomerates are formed when the fine oil particles cover the surface of the lipophilic and hydrophobic sludge. According to the dewatering method of the present invention, sludge can be effectively dewatered.
2. Description of the Related Art
In order to better understand the background of the present invention, conventional techniques and current technological situation concerning the treat of sewage sludge are provided below.
Unit processes currently used to treat and dispose of sewage sludge are classified into the following categories according to their purpose and functions.
Concentration => Anaerobic digestion => dewatering => => Drying → landfill, soil conditioner => Composting → soil conditioner, fertilizer for green agriculturalland after examining harmfulness => Earthworm feed → earthworm breeding, soil conditioner => Solidification → landfill, soil coverings => Incineration → landfill, construction materials (raw materialsof bricks and cement) using incinerated ash => Melting → construction materials (pavement materials,lightweight aggregates, blocks), landfill => Pyrolysis → fuels, e.g., gas, oil, char, etc. => Ground-landfill => Coastal-landfill => Ocean dumping
As explained above, the sludge treatments are divided into a pretreatment process, an intermediate treatment process and a final treatment process. The pretreatment process for the weight reduction of sludge includes concentration, digestion and mechanical dewatering. The intermediate treatment process for the weight reduction and stabilization of dewatered cakes includes composting, incineration, melting, solidification and the like. Most final waste products obtained after the final treatment process are commonly buried without drying, or reused. Leachate generated from the buried waste products becomes a serious problem. In addition, since environmental regulations have recently become more stringent for ground-landfill/coastal-landfill and ocean dumping, special attention has been paid to incineration processing.
Incineration is a process wherein combustible materials are burned using oxygen present in the air. In order to solve the problem of insufficient available landfill sites, weight reduction of waste materials and stabilization for treating septic matters by transforming organic materials into inorganic materials, are widely used as processes for treating waste materials. Since incineration processing is advantageous in that the waste heat released during incineration of waste materials can be reused for electricity generation and district heating, it has been used to treat municipal waste materials for a long time. In the case of hazardous waste materials, weight reduction of sludge is preferential over stabilization and safety effects because of limited landfill sites, particularly, in metropolitan areas. In most big cities of industrialized countries, conventional landfill processing is increasingly changed toward incineration processing.
The major benefits of incineration processing are sanitary disposal, no septicity, less disgust than dewatered cakes, and volume reduction to 10˜20% of the initial volume of sludge.
Since concentrated sludge generated from a sewage terminal treatment plant contains a water content of 95% or more, it needs to be dewatered for subsequent processing. The use of a general method for dewatering sludge by mechanical dewatering enables the water content to be reduced to 75˜85% (see, FIG. 1).
Following dewatering, the sludge is dried and then incinerated. The heat values of some types of sludge are shown in the following table.
Heating valuein total solids (kJ/kg)RepresentativeType of sludgeRangevalueRaw sludge23,000–29,00025,000Surplus sludge20,000–23,00021,000Anaerobically digested sludge 9,000–14,00012,000Chemically settled sludge14,000–18,00016,000Biologically treated sludge16,000–23,00020,000*1 kJ = 0.2389 kcal
On the other hand, the content of heavy metals in sludge depends largely on cities and seasons. Analytical data of heavy metals contained in typical municipal sewage sludge of an industrially advanced country are shown in the table below.
AverageAverageValuevalueElementRange (ppm)(ppm)ElementRange (ppm)(ppm)Ag<3–7  3K 920–1,9001,300Al 5,400–12,0009,000Li<2–7  2Ba47–50170Mg 880–7,4001,600Ca 5,900–17,0009,800Mn 50–240130Cd 2–229Na1,800–7,4004,500Co<3–5  3Ni 9–9022Cr 20–10055Pd  110–1,500330Cu 80–900350Sb20–4045Fe1,000–3,5002,300Sn<20–40  20Hg0.66–1.9 1.2Zn  200–2,500780
Centrifugal dewatering, vacuum dewatering using a partial pressure difference between phases and press dewatering are now commercially available to dewater sewage sludge, all of which are processes using a filter. In addition to these processes, a lagoon process wherein sludge in a concentration tank is subjected to solid-liquid separation by a sand filter layer and then dried in ambient air can be used to dewater sewage sludge. The advantages and drawbacks of the above-mentioned processes are explained in the table below.
DewateringprocessAdvantagesDrawbacksCentrifugal dewateringClean working environment, lessConsiderable abrasion ofmalodor and contamination, easyscroll → high maintenancestart/stop operationcostRelatively high dewatering ratePrevious removal of sandLow equipment cost compared toor hard inorganiccapacitymaterials requiredSmall occupied area compared toSkilled person forother equipmentsoperation requiredAppropriate filteringaid requiredContinuous-typeLess energy consumptionSevere malodorpress dewateringRelatively low-cost equipmentComplete removal of fineand operation costsludge and largeSimple mechanical structure andimpurities requiredeasy maintenanceSensitivity to sludgePossible high-pressure operationfeeding conditionsfor high dewatering rateCompletely automaticVery easy start and stopoperation not preferredoperationBatch-typeFormation of filter cakes havingDiscontinuous operationpress dewateringexcellent dewatering effects→ high labor costExcellent filtering effects andHigh equipment cost andfewer suspended substances insupport structuresfiltraterequiredLarge operation spaceand skilled personrequiredFiltering chemicalsrequired → further sludgegenerationSludgeLowest equipment cost, if spaceLarge area requireddrying bedis permittedDigested sludge requiredRelative insensitivity to sludgeSensitivity to weatherfeeding conditions → no skilland difficult operation inrequired to operatewinterLess energy consumption and useIntensive labor forof small amounts of chemicalsremoval of sludge andHigh content of solids, comparedcleaning of concentrationto mechanical processestank requiredSludge lagoonsLess energy consumption and noMalodor andneed for chemical additioncontamination of pathogensComplete digestion of organicLeachate generationmaterialsLarge area and poorLow equipment costs, if space isappearancepermitted, and no skill requiredSensitivity to weatherto operateand difficult operation inwinter
Meanwhile, in order to better improve the dewatering properties during dewatering of sludge or solid microorganisms, a chemical conditioning or stabilization treatment of sludge is performed. In addition to the chemical conditioning treatment, a physical heating treatment and a freeze-thaw treatment are used. However, since these treatments are economically disadvantageous in terms of their operation, they are limited to small-scale industrial processes.
The chemical conditioning treatment is evaluated to be the most economical sludge dewatering treatment, in terms of high recovery rate and compatibility with other treatments. Depending on the type of sludge to be treated, the chemical conditioning treatment can lower the water content of sludge from 90˜99% to 65˜85%. According to the chemical conditioning treatment, simple coagulation between sludge and sludge solids takes place, and as a result, absorbed water is discharged (dewatering). The chemical conditioning treatment is commonly used in processes, e.g., centrifugal dewatering, belt-filter press or pressure filter treatment, where a more elaborate dewatering is required. As chemicals used for the chemical conditioning treatment, iron chlorides, limestone, alum, polymeric materials, etc., are used. In the case that a polymeric material is used as a chemical for the chemical conditioning treatment, additional non-combustible inorganic sludge is not generated. However, in the case that an iron chloride or limestone is used as a chemical for the chemical conditioning treatment, 20˜30% of sludge based on dried sludge is further generated. The chemical for the chemical conditioning treatment is properly selected according to the concentration of entering sludge and reaction conditions of a reaction solution, for example, pH, alkalinity, reaction time. For instance, a large amount of limestone is required to dewater sludge to a high extent and to increase pH and alkalinity. At this time, a large amount of ammonia gas is evolved during sludge dewatering and additional sludge is generated. The choice of a chemical also depends on the dewatering processes employed, e.g., the polymeric material is mainly used in the centrifugal dewatering process and belt-filter press treatment, but is not suitable in the pressure filter treatment. The amount of entering sludge generally varies according to the characteristics of the entering sludge. As the dewatering of sludge becomes difficult, the amount of the chemical addition increases, the formation of dry cakes is difficult and filtering efficiency is poor. Generally, dewatering of untreated primary sludge is easiest. Mixed sludge containing the untreated primary sludge, anaerobically treated sludge and aerobic sludge follow the untreated primary sludge in terms of ease of dewatering.
As other conditioning treatments, heat treatment, preheating treatment, freeze-thaw treatment and the like can be mentioned. According to the heat treatment for sludge conditioning and stabilization, sludge is heated under pressure for a short time to destroy its gel-structure and to reduce its hydrophilicity. As a result of these heat-treating effects, even moisture present within microorganisms can be removed and the water content of dry cakes can be reduced to 30˜50%. In addition, since no additional conditioning treatment is required, the formation of sludge having a heating value of about 30 kJ/g is possible. However, the drying process has disadvantages that a large quantity of noxious gases, including ammonia, is evolved during heat-treating and a supernatant having a high BOD is formed. Accordingly, an additional secondary treatment of wastewater and air is indispensable to remove the noxious gases and supernatant. Furthermore, since equipment used for the heat treatment is very expensive, the heat treatment is partially used in a small-scale process.
The preheating treatment is based on the fact that the preheating of sludge to about 60° C. can increase the dewatering effects of sludge by about 6%. However, since the preheating treatment requires recirculation of filter-treated water having a high BOD, it can be effectively utilized in areas using waste heat.
According to the freeze-thaw treatment, repeated freezing and thawing changes the jelly-structured sludge into fine granules so that the filtering resistance is reduced and thus the sludge is effectively dewatered. The freeze-thaw treatment is ineffective to dewater bound water, but is effective for relatively hard-to-dewater materials. The dewatering operation of the freeze-thaw treatment is relatively easy, and the water content in cake residues can be reduced to 25˜40%.